Method of alignment for buried structures formed by surface transformation of empty spaces in solid state materials

ABSTRACT

A method of aligning a plurality of empty-spaced buried patterns formed in semiconductor monocrystalline substrates is disclosed. In an exemplary embodiment, high-temperature metal marks are formed to include a conductive material having a melting temperature higher than an annealing temperature used to form such empty-spaced buried patterns. The high-temperature metal marks are formed prior to the formation of the empty-spaced buried patterns formed in a monocrystalline substrate, so that the empty-space buried patterns are aligned to the marks. Subsequent semiconductor structures that are formed as part of desired semiconductor devices can be also aligned to the marks.

FIELD OF THE INVENTION

[0001] The present invention relates to semiconductor devices andmethods of making such devices. More particularly, the invention relatesto solid state materials and to a novel method of aligning buriedstructures formed in such solid state materials.

BACKGROUND OF THE INVENTION

[0002] The fabrication of integrated circuits (IC) devices onsemiconductor wafers include various steps during which patterns aretransferred from photolithographic masks on the wafers. The masking stepinvolves an etching step and defines predefined areas to be exposed onthe wafer for subsequent processing, for example, oxidation,metallization, or doping, among others. This photolithographic processis repetitively performed until desired patterns of materials are formedon the semiconductor wafer.

[0003] As the dimensions of these patterns become increasingly smaller,it is strictly required to accurately align the patterns previouslyformed on the semiconductor wafer with a pattern that is to besubsequently formed, and to minimize the misalignment between IC layers.To accurately carry out this process, the semiconductor industry employsalignment marks which are provided at predetermined position on thesurface of a wafer, so that relative positioning between patterns isperformed referring to these marks.

[0004] Typically, alignment marks are topographical patterns, such assquares, crosses or chevrons, among others, which are formed by etchinginto the wafer to provide slit patterns constituted of longitudinalindentations over specific intervals at semiconductor surface, aninsulating layer or other layers of a semiconductor substrate.

[0005] The formation of alignment marks is typically executedsimultaneously with, or after, other processes, for example theformation of metallization layers over a semiconductor substrate. Inthis case, the etching of the alignment mark must be conducted withextreme care, to avoid overetching of the substrate and/or of theunderlying layers, which could be metal layers. Another problem posed bythe formation of the alignment marks under these circumstances is thereadability of the mark, particularly when an oxide layer covers themark. As known in the industry, the formation of semiconductor devicesrequires, in most cases, a series of oxidation steps to form variousoxide layers at different stages of processing. For example, newisolation processes such as shallow trench isolation (STI) necessitate athick oxide layer formed over both the wafer and the alignment marks.When the thick oxide layer is later polished, typically by chemicalmechanical polishing, to create a planar surface, the alignment marks ona new overlying layer on the wafer are flattened after planarization,causing alignment target reading problems.

[0006] Another problem encountered by conventional alignment processesrelates to the alignment of various buried structures. As thesemiconductor industry is exploring new ways of increasing the amount ofactive surface area on the integrated circuit chips, particularly onthose employing monocrystalline semiconductor substrates, attempts tocreate and develop new technologies have been made continuously. Forexample, one technology proposed by the semiconductor industry is theso-called Silicon-On-Insulator (SOI) process, wherein oxygen atoms areimplanted at high dose and energy to form a silicon dioxide insulatinglayer between the upper surface of the original monocrystallinesubstrate and the bottom bulk portion of the same substrate. Althoughthe SOI devices have many advantages, such as reduced parasiticcapacitance due to the buried insulating layer, the process isrelatively expensive because of the high costs of implanting the oxygenatoms and curing of the implant-induced defects. Further, buriedstructures such as SOI devices are completely covered by the reformedmonocrystalline semiconductor substrate and thus they become essentiallynon-readable for alignment purposes.

[0007] Accordingly, there is a need for an improved method of increasingthe available active surface area on integrated circuit chips fabricatedon monocrystalline substrates by forming buried structures within suchsubstrates. There is also a need for a more advantageous alignmentprocess for such buried structures formed in monocrystallinesuperconducting substrates. There is further a need for an improvedmetallization scheme which facilitates the formation of active deviceson SOI substrates and on the more novel Silicon-On-Nothing (SON)substrates, as well as a need for accurate alignment of suchmetallization scheme with subsequently formed layers.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention provides a method of forming an alignmentmark for aligning a plurality of empty-spaced patterns formed insemiconductor monocrystalline substrates. According to an exemplaryembodiment, alignment metal marks are formed of a conductive materialhaving a melting temperature higher than the annealing temperature usedto form the empty-spaced patterns. The alignment marks are formed priorto the formation of empty-spaced buried patterns formed in amonocrystalline substrate, so that the empty-space buried patterns arealigned to the marks. Holes could be next formed in the monocrystallinesubstrate to connect surfaces of the substrate with the previouslyformed empty-space patterns. The whole assembly is subsequently exposedto an oxidizing atmosphere so that the inner surfaces of the empty-spacepatterns are oxidized. The empty-space patterns could then be filledwith a suitable conducting material to form, for example, buriedconductors and/or buried plate patterns. Subsequent semiconductorstructures that are formed as part of desired semiconductor devices canbe also aligned to the marks. This way, a proper alignment of thesemiconductor devices to the buried conductors and/or buried platepatterns is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 illustrates a cross-sectional view of a silicon substrateat a preliminary step of processing undertaking a sequence of steps forthe formation of an alignment mark in accordance with a method of thepresent invention.

[0010]FIG. 2 illustrates a cross-sectional view of the alignment mark ofFIG. 1 at a stage of processing subsequent to that shown in FIG. 1.

[0011]FIG. 3 illustrates a cross-sectional view of the alignment mark ofFIG. 1 at a stage of processing subsequent to that shown in FIG. 2.

[0012]FIG. 4 illustrates a cross-sectional view of the alignment mark ofFIG. 1 at a stage of processing subsequent to that shown in FIG. 3.

[0013]FIG. 5 illustrates a cross-sectional view of the alignment mark ofFIG. 1 at a stage of processing subsequent to that shown in FIG. 4.

[0014]FIG. 6 illustrates a top view of the alignment mark of FIG. 1 at astage of processing subsequent to that shown in FIG. 5.

[0015] FIGS. 7(a)-(f) illustrate a portion of a silicon substrateundertaking a sequence of steps for single-shaped sphere formation.

[0016] FIGS. 8(a)-(c) illustrate a portion of a silicon substrateundertaking a sequence of steps for single pipe-shaped empty spaceformation, performed in accordance with a method of aligning a buriedpattern of the present invention.

[0017] FIGS. 9(a)-(b) illustrate a portion of a silicon substrateundertaking a sequence of steps for plate-shaped empty space formation,performed in accordance with a method of aligning a buried pattern ofthe present invention.

[0018]FIG. 10 is a cross-sectional of the silicon structure of FIG. 8,taken along line 10-10″, at an intermediate stage of processing and inaccordance with a first embodiment of the invention, and depicting thealignment mark of FIG. 5.

[0019]FIG. 11 is a cross-sectional view of the silicon structure of FIG.10 according to the present invention at a stage of processingsubsequent to that shown in FIG. 10.

[0020]FIG. 12 is a cross-sectional view of the representative siliconstructure according to the present invention at a stage of processingsubsequent to that shown in FIG. 11.

[0021]FIG. 13 is a cross-sectional view of the representative siliconstructure according to the present invention at a stage of processingsubsequent to that shown in FIG. 12.

[0022]FIG. 14 is a cross-sectional view of the representative siliconstructure according to a second embodiment of the present invention at astage of processing subsequent to that shown in FIG. 12.

[0023]FIG. 15 is a cross-sectional view of the representative siliconstructure according to the present invention at a stage of processingsubsequent to that shown in FIG. 13.

[0024]FIG. 16 is a cross-sectional view of the representative siliconstructure according to the present invention at a stage of processingsubsequent to that shown in FIG. 15.

[0025]FIG. 17 is a three-dimensional view of the representative siliconstructure of FIG. 16.

[0026]FIG. 18 is a cross-sectional view of the representative siliconstructure according to the present invention at a stage of processingsubsequent to that shown in FIG. 16.

[0027]FIG. 19 is a cross-sectional view of the representative siliconstructure according to the present invention at a stage of processingsubsequent to that shown in FIG. 18.

[0028]FIG. 20 is a cross-sectional view of the representative siliconstructure according to the present invention at a stage of processingsubsequent to that shown in FIG. 19.

[0029]FIG. 21 is a schematic diagram of a processor system incorporatinga silicon structure with an alignment mark of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] In the following detailed description, reference is made tovarious exemplary embodiments for carrying out the invention. Theseembodiments are described with sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be employed, and that structural, electrical andprocess changes may be made, and equivalents substituted, withoutdeparting from the invention. Accordingly, the following detaileddescription is exemplary and the scope of the present invention isdefined by the appended claims.

[0031] The term “substrate” used in the following description includesany semiconductor-based structure having an exposed surface in which thestructure of this invention may be formed. The term “substrate” is to beunderstood as including substrates formed of silicon,silicon-on-insulator, doped and undoped semiconductors, epitaxial layersof silicon supported by a base semiconductor foundation, and othersemiconductor and dielectric structures. Furthermore, when reference ismade to a substrate in the following description, previous process stepsmay have been utilized to form regions or junctions in the basesemiconductor structure or foundation.

[0032] The following illustration is for a particular embodiment in asilicon structure. However, it should be apparent to one skilled in theart that a similar embodiment is possible in any semiconductorstructure.

[0033] Referring now to the drawings, where like elements are designatedby like reference numerals, FIGS. 1-21 illustrate exemplary embodimentsof a method of aligning buried silicon structures 100 comprising buriedconductor patterns formed in accordance with the present invention.FIGS. 1-6 illustrate the formation of a series of high-temperaturealignment marks 200 formed in a silicon substrate 10 according to anembodiment of the present invention. FIGS. 7-21 illustrate the formationof empty-spaced patterns in the silicon substrate 10, on which theburied conductor patterns of the present invention will be formed andaligned relative to the alignment marks 200.

[0034] Reference in now made to FIG. 1, which illustrates across-sectional view of a silicon substrate 10 within which squarealignment marks 200 (FIGS. 5-6) are formed according to exemplaryembodiments of the present invention. For simplicity, the method of thepresent invention will be described above with reference to theformation of only one square alignment mark 200, but it must beunderstood the invention contemplates the formation of a plurality ofsuch alignment marks. Further, although the present invention will bedescribed with reference to the formation of a square alignment mark, itmust be understood that the shape, geometry and configuration of thealignment mark is not limited to that of a square, and other geometries,such as crosses or chevrons, for example, may be used also.

[0035] As shown in FIG. 1, the silicon substrate 10 is patterned andetched by known photolithography techniques to form a trench 210 in thesilicon substrate 10. The trench 210 is etched to a depth of about 1,000Angstroms to about 10,000 Angstroms, more preferably of about 2,000Angstroms. The trench 210 is also etched to a width of about one-thirdof the depth, or of about 300 Angstroms to about 3,000 Angstroms, in theexample given.

[0036] Subsequent to the formation of the trench 210, the siliconsubstrate 100 is oxidized to thermally grow a silicon oxide layer 220into the trench 210 and over the surface of the silicon substrate 10, asillustrated in FIG. 2. The thermal oxidation of the silicon substrate 10could take place, for example, in an oxygen (O₂) or water (H₂O) vaporambient, at temperatures of about 750° C. to about 1000° C. depending onthe desired oxidation rate. This way, the silicon oxide layer 220 isformed to a thickness of about 100 Angstroms to about 500 Angstroms,more preferably of about 250 Angstroms. The silicon oxide layer 220 actsas a barrier layer to slow down the diffusion of metal atoms from asubsequently deposited metal film, the formation of which will bedescribed in more detail below. In addition, the silicon oxide layer 220also enhances the adherence of the metal film to the silicon substrate10.

[0037] Next, as illustrated in FIG. 3, a high-temperature conductivematerial layer 230 is formed overlying the silicon oxide layer 220 andfilling partially the trench 210. The conductive material layer 230 isformed to a thickness of about 50 Angstroms to about 500 Angstroms, morepreferably of about 100 Angstroms. The thickness of the conductivematerial layer 230 determines the dimensions of alignment edge D (FIGS.5-6). For example, if an alignment edge of about 100 Angstroms isdesired, then the thickness of the conductive material layer 230 isabout 100 Angstroms. In any event, the conductive material layer 230 maybe blanket deposited by a known PVD, CVD, or a combination of thesetechniques. Alternatively, the conductive material layer 230 may bedeposited by a plating technique or by using plasma or reactivesputtering.

[0038] In a preferred embodiment of the invention, the conductivematerial layer 230 is formed of a conductive metal with ahigh-temperature melting point. A characteristic of the conductivematerial layer 230 is that it must sustain temperatures higher than1100° C., which are necessary for the formation of the buried structures100 (FIG. 20) described in more detail below. For example, metals suchas iridium (Ir) or tungsten (W), which have high-temperature meltingpoints (melting point of Ir is about 2410° C. and that of W about 3410°C.±20° C.), are best candidates for the conductive material layer 230.If the preferred material for the conductive material layer 230 isiridium, for example, the diffusion of iridium atoms from the iridiumlayer 230 is prevented by the oxide layer 220 which acts as a barrierlayer as well as an adhesion layer.

[0039] Although iridium and tungsten are preferred, other metals orcombinations of such metals may be used also, as long as they retain amelting temperature higher than 1100° C. As such, molybdenum (Mo)(melting point of about 2617° C.), tantalum (Ta) (melting point of about2996° C.), platinum (Pt) (melting point of about 1772° C.), chromium(Cr) (melting point of about 1857° C.), nickel (Ni) (melting point ofabout 1453° C.), or combinations of these metals may be used also. Inany event, the term “high-temperature resistant alignment mark” as itwill be used in this application refers to an alignment mark whichincludes at least a conductive material with a melting temperaturehigher than an annealing temperature used to formed empty-spaced buriedpatterns in the substrate material by methods described in more detailbelow. Thus, for a monocrystalline silicon substrate, such as thesilicon substrate 10 of the present invention, a high-temperatureresistant alignment mark formed by a method of the present inventionincludes a conductive material capable of remaining intact attemperatures higher than about 1100° C., which, as it will be explainedbelow, is the annealing temperature for forming empty-spaced patterns ina monocrystalline silicon substrate. Such annealing temperature is, inturn, lower than the melting point of monocrystalline silicon, which isabout 1400° C.

[0040] Referring now to FIG. 4, an insulating layer 240 of, for example,silicon nitride (Si₃N₄) or an oxide such as silicon dioxide (SiO₂), isformed overlying the conductive material layer 230 and fillingcompletely the trench 210. The insulating layer 240 may be formed byconventional deposition methods to a thickness of about 100 Angstroms toabout 1,500 Angstroms, more preferably of about 300 Angstroms.

[0041] Subsequent to the formation of the insulating layer 240, portionsof the oxide layer 220, the conductive material layer 230, and theinsulating layer 240 that are formed overlying the silicon substrate 10are etched back by means of chemical mechanical polishing (CMP) orwell-known RIE dry etching processes. In a preferred embodiment,chemical mechanical polishing is employed, so that an abrasive polishremoves the top surface of the insulating layer 240 as well as thehorizontal portions of the oxide layer 220 and the conductive materiallayer 230 down to or near the planar surface of the surface of thesilicon substrate 10, as illustrated in FIG. 5. This way, ahigh-temperature resistant alignment mark 200 is formed on the siliconsubstrate 10 with an alignment edge D having a width determined by thethickness of the conductive material layer 230. For a betterunderstanding, a top view of the square alignment mark 200 isillustrated in FIG. 6.

[0042] Reference is now made to FIGS. 10-20, which illustrate exemplaryembodiments of buried silicon structures 100 comprising buried conductorpatterns formed within the silicon substrate 10 and aligned relative tothe square alignment mark 200 (FIGS. 5-6). To understand, however, theformation of such buried conductor patterns, reference is first made toFIGS. 7-10 which illustrate the formation of empty-space patterns onwhich the buried conductor patterns of the present invention will beformed and aligned. Techniques for the formation of empty-spacedpatterns of different geometries are described by Sato et al., inSubstrate Engineering for the Formation of Empty Space in Silicon (ESS)Induced by Silicon Surface Migration, 1999 IEDM Digest, Paper 20.6.1,the disclosure of which is incorporated by reference herein.

[0043] Empty spaces which are formed in silicon substrates and havevarious shapes, such as plates, spheres or pipes, may be formed as aresult of the self-organizing migration properties on the siliconsurface. As such, when deeply-etched silicon substrates are annealed ina hydrogen ambient, for example, the silicon atoms on the surfacemigrate so that their surface energy is minimized. Based on thesefindings, Sato et al. have demonstrated that the geometry of emptyspaces, such as spheres, plates and pipes, formed under the surface of asilicon substrate depends on the size, number and spacing of a pluralityof cylindrical holes that are initially formed at a low temperature.

[0044] For example, FIGS. 7(a)-(f) illustrate how a single sphere-shapedempty space 13 is formed from a single cylindrical hole 12 formed withinthe silicon substrate 10. Subsequent to the formation of the cylindricalhole 12, the silicon substrate 10 is annealed at a temperature lowerthan the melting point of monocrystalline silicon (1400° C.), forexample, at a temperature of about 1100° C. Sato et al. havedemonstrated that, within about 60 seconds and under a reducing ambientof 10 Torr of hydrogen, the shape and surface morphology of thecylindrical hole 12 changes drastically to that of the sphere-shapedempty space 13 (FIG. 7(f)). Because of the significant surface and/orvolume diffusion which occurs at high annealing temperatures, thecylindrical hole 12 is unstable beyond a critical length Lc andtransforms, therefore, to a lower energy state consisting of one or moreempty spheres formed along the original cylinder axis.

[0045] As analyzed by Nichols et al., in Surface- (Interface-) andVolume-Diffusion Contributions to Morphological Changes Driven byCapillarity, Trans. AIME 233 at 1840 (October 1965), the disclosure ofwhich is incorporated by reference herein, when Lc corresponds to thesurface diffusion state, the number N of empty spheres that form from acylindrical hole depends both on the length L of the cylindrical holeand on the cylinder radius Rc. Accordingly, the number N of emptyspheres formed from a cylindrical hole made in a silicon substrate couldbe estimated according to the following equation:

8.89Rc N<L<8.89 Rc (N+1)   (1)

[0046] wherein:

[0047] N=number of empty spheres;

[0048] Rc=cylinder radius; and

[0049] L=length of cylindrical hole

[0050] Thus, equation (1) predicts that, if L<8.89 Rc, the number ofempty spheres will be N=0, which means that no empty spheres will formfrom a cylindrical hole.

[0051] When one or more empty spheres form with a radius Rs, thenaccording to Nichols et al., the value of Rs is given by the followingequation:

Rs=1.88 Rc   (2)

[0052] wherein:

[0053] Rs=sphere radius; and

[0054] Rc=cylinder radius

[0055] When two or more empty spheres form from a cylinder hole with acylinder radius Rc, then the distance λ between the centers of twoadjacent empty-spaced spheres is calculated from the following formula:

λ=8.89 Rc   (3)

[0056] wherein:

[0057] λ=center-to-center distance between two adjacent spheres; and

[0058] Rc=cylinder radius

[0059] Reference is now made to FIGS. 8(a)-(c), which exemplify theformation of a single pipe-shaped empty space 23 from a linear array ofcylindrical holes 22. Similarly, FIGS. 9(a)-(b) illustrate the formationof a single plate-shaped empty space 33 from a two-dimensional array ofcylindrical holes 32 formed within a silicon substrate such as thesilicon substrate 10. The values of the pipe radius Rp (of thepipe-shaped empty space 23) and that of the plate thickness Tp (of theplate-shaped empty space 33) may be calculated in a manner similar tothat described above with reference to the formation of the empty sphere13 and the calculation of sphere radius Rs in equation (2). The distanceΔ between the centers of any two adjacent cylindrical holes 22, 32, in alinear array, may be calculated from the following formula:

2 Rc<Δ<3.76 Rc   (4)

[0060] wherein:

[0061] Rc=cylinder radius; and

[0062] Δ=center-to-center distance between two adjacent cylinder holesin a linear array

[0063] Equation (4) ensures that adjacent cylindrical holes 22, 32 donot touch each other allowing, therefore, the formation of a pluralityof adjacent spheres that combine to form the resulting pipe-shaped emptyspace 23 and plate-shaped empty space 33.

[0064] The values of the pipe radius Rp and of the plate thickness Tpare given by the following two expressions:

Rp=(8.86 Rc ³/Δ)^(1/2)   (5)

Tp=27.83 Rc ³/Δ²   (6)

[0065] wherein:

[0066] Rp=pipe radius;

[0067] Tp=plate thickness; and

[0068] Δ=center-to-center distance between two adjacent cylinder holesin a linear array

[0069] Reference is now made to FIG. 10 which, for simplicity,illustrates a cross-sectional view of structure of FIG. 8(a) on which aplurality of linear cylindrical holes 22 are drilled into siliconsubstrate 10 which also includes high-temperature alignment marks, suchas the alignment mark 200 (FIGS. 5-6), the formation of which wasdescribed above. The linear cylindrical holes 22 are drilled intosilicon substrate 10 from an upper surface 11 of the substrate 10 to adepth D1. The silicon substrate 10 is annealed at a temperature of about1100° C. and under a reducing ambient of about 10 Torr of hydrogen sothat within about 60 seconds a pipe-shaped empty space 23 is formedwithin the silicon substrate 10 as shown in FIG. 11.

[0070] Radius R1 (FIG. 10) of each of the cylindrical holes 22 anddistance Δ1 (FIG. 10) between the center of two adjacent cylindricalholes 22 may be calculated in accordance with equation (4). It must beunderstood that the length L1 (FIG. 10) of the array of the cylindricalholes 22 determines the length L1 (FIG. 11) of the pipe-shaped emptyspace 23, wherein the depth D1 (FIG. 10) to which the array ofcylindrical holes 22 is drilled determines the depth D1 (FIG. 11) atwhich the pipe-shaped empty space 23 is formed. Both parameters define alocation where a first level conductor 70 (FIGS. 19-20), will be formedas described in more detail below. Finally, the radius Rp of thepipe-shaped empty space 23 (FIG. 11) may be calculated in accordancewith equation (5).

[0071] Subsequent to the formation of the pipe-shaped empty space 23, asecond pipe-shaped empty space 43 (FIG. 13) may be formed above thepipe-shaped empty space 23 and below the silicon surface 11 by atechnique similar to that described for the formation of the pipe-shapedempty space 23 (FIG. 11). As such, a second linear array of cylindricalholes 42 (FIG. 12) are drilled into the silicon substrate 10 to a depthD2 to define the intended location, length and orientation of a secondlevel conductor 80 (FIGS. 19-20), the formation of which will bedescribed in more detail below. The relative positioning between thepipe-shaped empty space 23 (FIG. 11) and the second linear array ofcylindrical holes 42 (FIG. 12) is accomplished by reference to thealignment mark 200. This way, the second linear array of cylindricalholes 42 is accurately aligned to the already formed pipe-shaped emptyspace 23.

[0072] The silicon substrate 10 is then annealed at a temperature ofabout 1100° C. and under a reducing ambient of about 10 Torr ofhydrogen, so that within about 60 seconds the second linear array ofcylindrical holes 42 transforms into the second pipe-shaped empty space43 (FIG. 13) by steps similar to those described above with reference toFIGS. 8(a)-(c).

[0073] Radius R2 (FIG. 12) as well as distance Δ2 (FIG. 12) between thecenter of two adjacent cylindrical holes 42 of the second linear arraymay be calculated in accordance to equation (4). Further, the length L2(FIG. 12) of the second linear array of cylindrical holes determines thelength L2 (FIG. 13) of the second pipe-shaped empty space 43, whereinthe depth D2 (FIG. 12) to which the second linear array of cylindricalholes is drilled determines the depth D2 (FIG. 13) at which the secondpipe-shaped empty space 43 is formed within the silicon substrate 10.

[0074] Although FIG. 13 illustrates the second pipe-shaped empty space43 as being parallel to the pipe-shaped empty space 23, it must beunderstood that the second pipe-shaped empty space 43 need not beparallel to the pipe-shaped empty space 23 but may form various anglesand may be placed in various directions with respect to the pipe-shapedempty space 23, according to the characteristics of the IC device to beformed. For example, FIG. 14 illustrates the second pipe-shaped emptyspace 43 forming a 90 degree angle with the pipe-shaped empty space 23.

[0075]FIG. 15 illustrates the formation of a two-dimensional array ofcylindrical holes 52 located in between the upper silicon surface 11 andthe second pipe-shaped empty space 43 which will form a plate-shapedempty space 53 (FIG. 16) aligned relative to the alignment mark 200.Again, the silicon substrate 10 is annealed at a temperature of about1100° C. and under a reducing ambient of about 10 Torr of hydrogen, sothat within about 60 seconds the two-dimensional array of cylindricalholes 52 transforms into the plate-shaped empty space 53 (FIG. 16) bysteps similar to those described above with reference to FIGS. 9(a)-(b).For a better understanding of the invention, the structures of FIG. 16are illustrated in a three-dimensional view in FIG. 17.

[0076] Radius R3 (FIG. 15) as well as distance Δ3 (FIG. 15) between thecenter of two adjacent cylindrical holes 52 of the two-dimensional arraymay be calculated in accordance to equation (4). Further, the length L3(FIG. 15) of the two-dimensional array of cylindrical holes determinesthe length L3 (FIG. 16) of the plate-shaped empty space 53, wherein thedepth D3 (FIG. 15) to which the two-dimensional array of cylindricalholes is drilled determines the depth D3 (FIGS. 16-17) at which theplate-shaped empty space 53 is formed within the silicon substrate 10.Finally, the thickness Tp (FIGS. 16-17) of the plate-shaped empty space53 may be calculated in accordance with equation (6). This plate-shapedempty region may be left empty in some areas, so that the region abovethe plate becomes a silicon-over-nothing area where various IC devicescan then be formed. Alternatively, the plate-shaped empty region may bealso filled with a conductor, as it will be described in more detailbelow, to provide a plate-shaped conductor region.

[0077] Subsequent to the formation of the first pipe-shaped empty space23, second pipe-shaped empty space 43, and plate-shaped empty space 53,additional interconnect structures and associated dielectric layerscould be formed to create operative electrical paths down from theempty-spaced structures formed within the silicon substrate 10 and up tothe silicon surfaces, such as the upper silicon surface 11, and any ICdevices formed thereon. These additional interconnect structures andassociated dielectric layers are also aligned with the previous emptyspace structures 23, 43, 53 by reference to the alignment mark 200. Forexample, the aligning method of the present invention is furtherexplained below with reference to the formation and alignment of buriedconductor patterns 70, 80, 90 (FIG. 20) relative to the alignment mark200.

[0078] Accordingly, as illustrated in FIG. 18, a plurality ofinterconnect holes 25, 45, 55 are drilled within the silicon substrate10 to connect each of the first pipe-shaped empty space 23, the secondpipe-shaped empty space 43, and the plate-shaped empty space 53 with theupper silicon surface 11. The structure of FIG. 18 is then subjected toa conventional oxidizing atmosphere, for example an ozone oxidizingatmosphere or water vapor is heated and passed over the substrate atabout 600-700° C., so that the inner surfaces of the above-describedpatterns, as well as the interconnect holes 24, 45, 55, are oxidized toprevent any leakage between conductors formed in areas 23, 43, 53, thesubstrate and any active devices that will be eventually fabricated overthe silicon substrate 10.

[0079] Alternatively, or in addition to the oxidizing atmosphere, abarrier layer 72 may be formed on surfaces of the silicon substrate 10,within each of the interconnect holes 24, 45, 55, and on the innersurfaces of each of the first pipe-shaped empty space 23, the secondpipe-shaped empty space 43, and the plate-shaped empty space 53, as alsoshown in FIG. 18. The barrier layer 72 may be formed by CVD, PVD,sputtering or evaporation, to a thickness of about 50 Angstroms to about100 Angstroms.

[0080] Preferred materials for the barrier layer 72 are metals, such astitanium (Ti), zirconium (Zr), tungsten (W), or hafium (Hf), or metalcompounds, such as tantalum nitride (TaN) or silicon nitride (Si₃N₄). Ifdesired, the barrier layer 72 may be formed of refractory metalcompounds, such as refractory metal nitrides (for example TiN and HfN),refractory metal carbides (for example TiC or WC), or refractory metalborides (for example TiB or MoB). The selection of a material for thebarrier layer 72 depends upon the specific conductor to be deposited andthe method which is chosen to deposit such conductor. In turn, theselection of the conductor material will depend upon the type andtemperature of subsequent processing steps. For example, aluminum (Al)would not be chosen as the conductor material if the subsequentprocessing steps require temperatures above approximately 600° C.Similarly, tungsten (W) would be a preferred conductor for temperaturesabove approximately 1000° C. In any event, the barrier layer 72suppresses the diffusion of the metal atoms from the subsequentlydeposited conductive material (FIG. 20), while offering a lowresistivity and low contact resistance between the subsequentlydeposited conductive material (FIG. 20) and the barrier layer 72.

[0081] Although in an exemplary embodiment of the invention the barrierlayer 72 is simultaneously deposited in the interconnect holes 24, 45,55, and on the inner surfaces of each of the first pipe-shaped emptyspace 23, the second pipe-shaped empty space 43, and the plate-shapedempty space 53, the invention is not limited to this embodiment. Forexample, the barrier layer 72 may be deposited first in the interconnecthole 24 and its corresponding pipe-shaped empty space 23, before theformation of the interconnect hole 45 and its corresponding secondpipe-shaped empty space 43, and before the formation of the interconnecthole 55 and its corresponding plate-shaped empty space 53. In thisembodiment, the barrier layer 72 may be formed of a first barriermaterial corresponding to the pipe-shaped empty space 23, of a secondbarrier material corresponding to the second pipe-shaped empty space 43,and of a third barrier material corresponding to the plate-shaped emptyspace 53. The first, second and third barrier materials may be the sameor different, depending on the characteristics of the IC device.

[0082] As illustrated in FIG. 19, a conductive material 73 is nextdeposited to fill in the interconnect holes 24, 45, 55, as well as thefirst pipe-shaped empty space 23, the second pipe-shaped empty space 43,and the plate-shaped empty space 53. In a preferred embodiment of theinvention, the conductive material 73 comprises either copper, silver,gold, tungsten or aluminum, but it must be understood that otherconductive materials and/or their alloys may be used also. In any event,the conductive material 73 may be blanket deposited by a substitutiontechnique, as described in U.S. Pat. Nos. 5,920,121; 6,100,176;6,121,126, and U.S. application Ser. No. 09/069,346 filed Apr. 29, 1998(disclosure of which is incorporated by reference). Alternatively, theconductive material 73 may be also blanket deposited by a known PVD,CVD, or a combination of these techniques to fill in all threeinterconnect holes 24, 45, 55 and their associated first pipe-shapedempty space 23, second pipe-shaped empty space 43, and plate-shapedempty space 53 to form a first buried conductor pattern 70, a secondburied conductor pattern 80, and a third buried conductor pattern 90,all illustrated in FIG. 20. Alternatively, the conductive material 73may be deposited by a plating technique.

[0083] After the deposition of the conductive material 73, excessbarrier material and excess metal formed above the upper silicon surface11 and over the alignment mark 200 may be removed by either an etchingor a polishing technique to form the buried silicon structure 100illustrated in FIG. 20. In an exemplary embodiment of the presentinvention, chemical mechanical polishing (CMP) is used to polish awayexcess barrier and conductive materials above the upper silicon surface11 of the silicon substrate 10.

[0084] Although the buried silicon structure 100 is shown in FIG. 20 ascomprising only three buried conductor patterns 70, 80, and 90,respectively, it must be readily apparent to those skilled in the artthat in fact any number of such buried conductor patterns may be formedon the substrate 10, as pipes, plates, or spheres and aligned relativeto the alignment mark 200, by methods of the present invention. Also,although the exemplary embodiments described above refer to theformation of buried conductor patterns having specific shapes, it mustbe understood that other shapes, configurations or geometries may beemployed, depending on the characteristics of the particular IC deviceto be fabricated and as long as they are accurately aligned with thepreviously formed patterns. Further, the invention is not limited to acombination of three buried conductor patterns, but any combination ofany number of empty-spaced patterns filled with a conductor may beemployed, as desired.

[0085] The processing steps of the present invention may be also reducedif the lower level buried conductor patterns do not cross over eachother below upper level buried patterns, such as the plate-shaped emptyspace 53 (FIGS. 16-18). In this case, all buried conductor patternslocated below the plate-shaped empty space 53 may be simultaneouslyaligned and formed during the same annealing/heating cycle, to reducetherefore the number of processing steps during the fabrication of theburied silicon structure 100.

[0086] In addition, further steps to create a functional memory cell onthe silicon substrate 10 may be carried out. Thus, additional multilevelinterconnect layers and associated dielectric layers could be formed tocreate operative electrical paths from the buried silicon structure 100to a source/drain region (not shown) adjacent to a transistor gatestructure (not shown) of the substrate 10. These additional multilevelinterconnect layers and associated dielectric layers may be alsopositioned and aligned with the buried conductor patterns 70, 80, 90(FIG. 20) with reference to the mark 200, to minimize misalignmentbetween the IC layers. The resulting substrate containing the buriedconductors can be used in the formation of many types of integratedcircuits such as memories, for example, DRAMs, processors etc. Thus, theinvention provides a technique for forming buried conductors insemiconductor substrates which are aligned relative to anhigh-temperature alignment mark and may be further used, for example, asinterconnects to various structures and devices in an integratedcircuit.

[0087] A typical processor-based system 400 which includes a memorycircuit 448, for example a DRAM, is illustrated in FIG. 21. A processorsystem, such as a computer system, generally comprises a centralprocessing unit (CPU) 444, such as a microprocessor, a digital signalprocessor, or other programmable digital logic devices, whichcommunicates with an input/output (I/O) device 446 over a bus 452. Thememory 448 communicates with the system over bus 452.

[0088] In the case of a computer system, the processor system mayinclude peripheral devices such as a floppy disk drive 454 and a compactdisk (CD) ROM drive 456 which also communicate with CPU 444 over the bus452. Memory 448, the CPU 444 or others of the illustrated electricalstructures may be constructed as an integrated circuit, which includesone or more buried silicon structures 100 aligned relative to thehigh-temperature alignment mark 200 in accordance with the invention. Ifdesired, the memory 448 may be combined with the processor, for exampleCPU 444, in a single integrated circuit.

[0089] The above description and drawings are only to be consideredillustrative of exemplary embodiments which achieve the features andadvantages of the present invention. Modification and substitutions tospecific process conditions and structures can be made without departingfrom the spirit and scope of the present invention. Accordingly, theinvention is not to be considered as being limited by the foregoingdescription and drawings, but is only limited by the scope of theappended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of forming a semiconductor device,comprising the steps of: forming at least one alignment mark inassociation with a semiconductor substrate; and forming a plurality ofempty-spaced patterns beneath a surface of, and within, saidsemiconductor substrate, said empty-spaced patterns being surrounded bysemiconductor material, said forming of said empty-spaced patternsincluding the aligning of said empty-spaced patterns to said alignmentmark.
 2. The method of claim 1, further comprising the act of forming aplurality of openings within said semiconductor substrate, said openingsconnecting a respective empty-spaced pattern with the exterior of saidsemiconductor substrate.
 3. The method of claim 2, further comprisingthe act of forming a conductive material in said empty-spaced patternsand said openings.
 4. The method of claim 1, wherein said act of formingsaid alignment mark further comprises forming a trench within saidsemiconductor substrate.
 5. The method of claim 4, wherein said act offorming said alignment mark further comprises etching said semiconductorsubstrate to a depth of about 1,000 Angstroms to about 10,000 Angstromsto form said trench.
 6. The method of claim 4, wherein said act offorming said alignment mark further comprises forming an oxide layeroverlying said semiconductor substrate and inside said trench.
 7. Themethod of claim 6, wherein said oxide layer is formed by oxidizing saidsemiconductor substrate.
 8. The method of claim 6, wherein said act offorming said at least one alignment mark further comprises forming aconductive material layer over said oxide layer, said conductivematerial layer having a melting temperature higher than an annealingtemperature used to form said plurality of empty-spaced patterns.
 9. Themethod of claim 8, wherein said conductive material layer is formed bydeposition to a thickness of about 50 Angstroms to about 500 Angstroms.10. The method of claim 8, wherein said conductive material layer isformed of a material selected from the group consisting of iridium,iridium alloy, tungsten, tungsten alloy, molybdenum, molybdenum alloy,nickel, nickel alloy, tantalum, tantalum alloy, chromium, chromiumalloy, platinum, and platinum alloy.
 11. The method of claim 8, whereinsaid conductive material layer is formed of iridium or iridium alloy.12. The method of claim 8, wherein said conductive material layer isformed of tungsten or tungsten alloy.
 13. The method of claim 8, whereinsaid act of forming said alignment mark further comprises forming aninsulating layer over said conductive material layer.
 14. The method ofclaim 13, wherein said insulating layer is an oxide layer.
 15. Themethod of claim 13, wherein said insulating layer is a nitride layer.16. The method of claim 13, wherein said act of forming said alignmentmark further comprises removing portions of said insulating layer, saidconductive material layer, and said oxide layer from said surface ofsaid semiconductor substrate.
 17. The method of claim 1, wherein saidact of forming said empty-spaced patterns further comprises forming atleast one hole within said semiconductor substrate and annealing saidsubstrate to form said empty-spaced patterns beneath said surface ofsaid semiconductor material.
 18. The method of claim 17, wherein saidhole is a cylindrical hole.
 19. The method of claim 17, wherein saidsemiconductor substrate is annealed at a temperature of about 1100° C.20. The method of claim 17, wherein said semiconductor substrate isannealed under a hydrogen ambient.
 21. The method of claim 17, whereinsaid semiconductor substrate is annealed for about 60 seconds.
 22. Themethod of claim 1, wherein at least one of said empty-spaced patternshas a pipe-shaped configuration.
 23. The method of claim 1, wherein atleast one of said empty-spaced patterns has a spherical configuration.24. The method of claim 1, wherein at least one of said empty-spacedpatterns has a plate-shaped configuration.
 25. The method of claim 3,wherein said empty-spaced pattern comprise at least one pipe-shapedpattern and at least one plate-shape pattern.
 26. The method of claim25, wherein said at least one pipe-shaped pattern and said at least oneplate-shape pattern are formed simultaneously.
 27. The method of claim25, wherein said at least one pipe-shaped pattern and said at least oneplate-shape pattern are formed sequentially and before said act offorming said conductive material.
 28. The method of claim 1, whereinsaid at least one alignment mark is formed over said semiconductorsubstrate.
 29. The method of claim 1, wherein said at least onealignment mark is formed within said semiconductor substrate.
 30. Themethod of claim 1, wherein said semiconductor substrate is a siliconsubstrate.
 31. The method of claim 1, wherein said semiconductorsubstrate is a germanium substrate.
 32. The method of claim 1, whereinsaid semiconductor substrate is a silicon-on-insulator substrate. 33.The method of claim 1, wherein said semiconductor substrate is asilicon-on-nothing substrate.
 34. A method of aligning at least oneburied conductor pattern in a monocrystalline substrate, said methodcomprising the steps of: forming at least one alignment mark inassociation with said monocrystalline substrate; forming a plurality ofcylindrical holes in said monocrystalline substrate, said cylindricalholes being aligned with respect to said at least one alignment mark;annealing said monocrystalline substrate to form at least oneempty-spaced pattern within, and surrounded by, said monocrystallinesubstrate and below a surface of said monocrystalline substrate; formingat least one opening in said monocrystalline substrate whichcommunicates with a respective empty-spaced pattern; and forming aconductive material in said empty-spaced pattern to form said at leastone buried conductor pattern.
 35. The method of claim 34, wherein saidact of forming said at least one alignment mark further comprisesforming a trench within said monocrystalline substrate.
 36. The methodof claim 34, wherein said act of forming said at least one alignmentmark further comprises etching said monocrystalline substrate to a depthof about 1,000 Angstroms to about 10,000 Angstroms to form said trench.37. The method of claim 34, wherein said act of forming said at leastone alignment mark further comprises forming an oxide layer overlyingsaid monocrystalline substrate and inside said trench.
 38. The method ofclaim 37, wherein said oxide layer is formed by oxidizing saidmonocrystalline substrate and said trench.
 39. The method of claim 37,wherein said act of forming said alignment mark further comprisesforming a conductive material layer over said oxide layer, saidconductive material layer having a melting temperature higher than anannealing temperature used to form said at least one empty-spacedpattern.
 40. The method of claim 39, wherein said conductive materiallayer is formed by deposition.
 41. The method of claim 39, wherein saidconductive material layer is formed of a material selected from thegroup consisting of iridium, iridium alloy, tungsten, tungsten alloy,molybdenum, molybdenum alloy, nickel, nickel alloy, tantalum, tantalumalloy, chromium, chromium alloy, platinum, and platinum alloy.
 42. Themethod of claim 39, wherein said conductive material layer is formed ofiridium or iridium alloy.
 43. The method of claim 39, wherein saidconductive material layer is formed of tungsten or tungsten alloy. 44.The method of claim 39, wherein said act of forming said at least onealignment mark further comprises forming an insulating layer over saidconductive material layer.
 45. The method of claim 44, wherein saidinsulating layer is an oxide layer.
 46. The method of claim 44, whereinsaid insulating layer is a nitride layer.
 47. The method of claim 44,wherein said act of forming said alignment mark further comprisesremoving any portions of said insulating layer, said conductive materiallayer, and said oxide layer from said surface of said monocrystallinesubstrate.
 48. The method of claim 34, wherein said act of annealing isperformed under a reducing atmosphere.
 49. The method of claim 48,wherein said reducing atmosphere is a hydrogen atmosphere at atemperature lower than the melting temperature of said monocrystallinesubstrate.
 50. The method of claim 49, wherein said act of annealing isperformed for about 60 seconds.
 51. The method of claim 34, wherein saidempty-space pattern is a pipe-shaped pattern.
 52. The method of claim34, wherein said empty-space pattern is a plate-shaped pattern.
 53. Themethod of claim 34, wherein said empty-space pattern is a sphere-shapedpattern.
 54. The method of claim 34, further comprising forming at leasttwo empty-spaced patterns.
 55. The method of claim 54, wherein said atleast two empty-spaced patterns are simultaneously formed.
 56. Themethod of claim 55, wherein one of said empty-spaced patterns is locatedbelow said other empty-spaced pattern relative to said surface of saidmonocrystalline substrate.
 57. The method of claim 34 further comprisingforming a barrier layer in said opening and said empty-spaced patternbefore said act of forming said conductive material.
 58. The method ofclaim 57 further comprising oxidizing said opening and said empty-spacedpattern before said act of forming said barrier layer.
 59. The method ofclaim 57 further comprising oxidizing said opening and said empty-spacedpattern before said act of forming said conductive material.
 60. Themethod of claim 34, wherein said act of forming said conductive materialfurther comprises depositing a conductive material inside saidempty-spaced pattern through said opening.
 61. The method of claim 60,wherein said conductive material is formed of a material selected fromthe group consisting of copper, copper alloy, silver, silver alloy,gold, gold alloy, tungsten, tungsten alloy, aluminum, and aluminumalloy.
 62. The method of claim 61 further comprising chemical mechanicalpolishing said substrate to remove said conductive material on a surfacethereof.
 63. The method of claim 34, wherein said monocrystallinesubstrate is a silicon substrate.
 64. The method of claim 34, whereinsaid monocrystalline substrate is a germanium substrate.
 65. The methodof claim 34, wherein said monocrystalline substrate is asilicon-on-insulator substrate.
 66. The method of claim 34, wherein saidmonocrystalline substrate is a silicon-on-nothing substrate.
 67. Amethod of aligning buried structures within a silicon substrate, saidmethod comprising the steps of: forming at least one alignment mark inassociation with said silicon substrate; forming a plurality ofcylindrical holes in said silicon substrate, said cylindrical holesbeing successively aligned with respect to said at least one alignmentmark; annealing said silicon substrate to form at least one empty-spacedpattern within, and surrounded by, said silicon substrate and below asurface of said silicon substrate; forming at least one opening in saidsilicon substrate which communicates with a respective empty-spacedpattern; and forming a conductive material in said empty-spaced pattern.68. The method of claim 67, wherein said act of forming said at leastone alignment mark further comprises forming a trench within saidsilicon substrate.
 69. The method of claim 68, wherein said act offorming said at least one alignment mark further comprises etching saidsilicon substrate to a depth of about 1,000 Angstroms to about 10,000Angstroms to form said trench.
 70. The method of claim 68, wherein saidact of forming said at least one alignment mark further comprisesforming an oxide layer overlying said silicon substrate and inside saidtrench.
 71. The method of claim 70, wherein said oxide layer is formedby oxidizing said silicon substrate and said trench.
 72. The method ofclaim 70, wherein said act of forming said at least one alignment markfurther comprises forming a conductive material layer over said oxidelayer, said conductive material layer having a melting temperaturehigher than an annealing temperature used for said act of annealing saidsilicon substrate.
 73. The method of claim 72, wherein said conductivematerial layer is formed by deposition.
 74. The method of claim 72,wherein said conductive material layer is formed of a conductivematerial having a melting temperature higher than 1100° C.
 75. Themethod of claim 74, wherein said conductive material layer is formed ofa material selected from the group consisting of iridium, iridium alloy,tungsten, tungsten alloy, molybdenum, molybdenum alloy, nickel, nickelalloy, tantalum, tantalum alloy, chromium, chromium alloy, platinum, andplatinum alloy.
 76. The method of claim 74, wherein said conductivematerial layer is formed of iridium or iridium alloy.
 77. The method ofclaim 74, wherein said conductive material layer is formed of tungstenor tungsten alloy.
 78. The method of claim 72, wherein said act offorming said at least one alignment mark further comprises forming aninsulating layer over said conductive material layer.
 79. The method ofclaim 78, wherein said insulating layer is an oxide layer.
 80. Themethod of claim 78, wherein said insulating layer is a nitride layer.81. The method of claim 78, wherein said act of forming said at leastone alignment mark further comprises removing any portions of saidinsulating layer, said conductive material layer, and said oxide layerfrom said surface of said silicon substrate.
 82. The method of claim 67,wherein said at least one alignment mark is formed within said siliconsubstrate.
 83. The method of claim 67, wherein said at least onealignment mark is formed on said silicon substrate.
 84. An integratedcircuit substrate comprising at least one empty-spaced pattern providedbeneath a surface of, and within, a semiconductor substrate, saidempty-spaced pattern being aligned with respect to an alignment mark.85. The integrated circuit of claim 84, wherein said empty-spacedpattern has a pipe-shaped pattern.
 86. The integrated circuit of claim84, wherein said empty-spaced pattern has a plate-shaped pattern. 87.The integrated circuit of claim 84, wherein said empty-spaced patternhas a spherical pattern.
 88. The integrated circuit of claim 84, whereinsaid alignment mark includes a conductive material with a melting pointhigher than an annealing temperature used to form said at least oneempty-spaced pattern.
 89. The integrated circuit of claim 84, whereinsaid semiconductor substrate is a silicon substrate.
 90. The integratedcircuit of claim 84, wherein said semiconductor substrate is a germaniumsubstrate.
 91. The integrated circuit of claim 84, wherein saidsemiconductor substrate is a silicon-on-insulator substrate.
 92. Theintegrated circuit of claim 84, wherein said semiconductor substrate isa silicon-on-nothing substrate.
 93. A processor system comprising: aprocessor; and a circuit coupled to said processor, at least one of saidcircuit and processor comprising: at least one alignment mark formedwithin a substrate; and a plurality of empty- spaced patterns formed byannealing said substrate having at least one hole drilled therein, saidempty space patterns being aligned with respect to said at least onealignment mark.
 94. The processor system of claim 93, wherein at leastone of said empty-spaced patterns has a configuration selected from thegroup consisting of pipe-shaped configuration, sphere-shapedconfiguration and plate-shaped configuration.
 95. The processor systemof claim 93, wherein at least one of said empty-spaced patterns has aplate-shaped configuration.
 96. The processor system of claim 93,wherein at least one of said empty-spaced patterns has a sphere-shapedconfiguration.
 97. The processor system of claim 93, wherein said atleast one alignment mark includes a conductive material having a meltingpoint higher than an annealing temperature used to form said pluralityof empty-spaced patterns.
 98. The processor system of claim 93, whereinsaid substrate is a monocrystalline substrate.
 99. The processor systemof claim 93, wherein said circuit is a memory circuit.
 100. Theprocessor system of claim 93, wherein said circuit is a DRAM memorycircuit.
 101. The processor system of claim 93, wherein said circuit andsaid processor are integrated on same circuit.
 102. The processor systemof claim 93, wherein said processor comprises said at least onealignment mark.
 103. The processor system of claim 93, wherein saidcircuit comprises said at least one alignment mark.